Digital Video Broadcasting has developed digital broadcast standards for cable, terrestrial, and satellite transmission. In 2003, the second generation digital video broadcasting standard for satellite transmission (DVB-S2) was developed and ratified by the European Telecommunications Standards Institute (ETSI). One of the features of the DVB-S2 standard is backward compatibility with DVB-S, the first generation standard. DVB-S2 also includes adaptive coding and modulation, LDPC coding together with an outer BCH code, and several coding rates. A further feature is four modulation modes (QPSK, 8PSK, 16APSK, and 32APSK). Future generations of the standards may expand on these key features and it is conceivable that backward compatibility will be included.
Normally, symbol timing recovery (STR) is done before the phase and frequency offset of the carrier is recovered using a feedback STR loop. Due to the problem of phase margin loss caused by delay in the STR loop, the error estimation of the symbol timing recovery is normally done before the matched filter in the receiver to minimize delay in the loop. For pulse shaping it is a common practice to use root raised cosine (RRC) filter to shape the transmitted pulse. This causes intersymbol interference (ISI) in the received signal before the matched filter, since an RRC pulse is not in the class of Nyquist impulses. As the roll-off factor decreases, the ISI gets more severe, and the acquisition range or pull-in of the feedback solution decreases.
Another problem arises for faster than Nyquist (FTN) signaling, where the pulse shaped symbols are shifted in time closer together. FTN will become a major topic in the coming years since bandwidth is limited and FTN provides another dimension in receiver design besides the usual bandwidth and excess bandwidth for a given SNR and bit error rate. FTN, by using information pulses on a temporally compressed timing grid, causes a huge amount of ISI and the classical timing recovery based on the feedback solution fails for large timing offsets. The coarse symbol timing recovery method described herein solves the problem of decreasing acquisition range as ISI gets more severe and provides a very robust recovery method, even to the ISI caused by FTN transmission.
Timing offset estimation in satellite receivers is normally done using a classical feedback solution with a Gardner timing error detector. Other solutions are also known, like the M&M detector, which uses post matched filtered samples and which is more insensitive to ISI but is very sensitive to phase and frequency errors.
The coarse outer timing estimate can be followed by a loop for fine symbol timing recovery, or each idea may be implemented separately.
The fine symbol timing recovery methods discussed herein help the symbol timing recovery system lock under adverse conditions, including low SNR, low excess bandwidth, and when faster than Nyquist (FTN) signaling is used. To squeeze more capacity from existing satellite channels, new methods are being explored to fit more data into the available bandwidth, such as reducing excess bandwidth, using faster than Nyquist signaling, and operating at lower SNR thresholds. Each of these approaches creates difficulties in symbol timing recovery. Symbol timing recovery was one of the limiting factors to the adoption of these capacity enhancing options.
These problems and others are addressed by the principles of the present invention in which a coarse and a fine symbol timing recovery scheme are presented. A novel symbol timing offset detection algorithm for data aided symbol timing estimation is used for coarse timing adjustment using pre-known data embedded in a DVB-S2 like transmission stream, which may be followed by a method for fine symbol timing recovery. Feed forward recovery relies on pre-known data symbols (e.g. pilot or sync symbols) embedded in the data stream. The coarse timing recovery idea described herein solves the problem of a traditional feeback approach of decreasing acquisition range as ISI gets more severe. It achieves this solution by using a data aided feed forward symbol timing recovery method that provides a very robust recovery method, even to the ISI caused by FTN transmission.
One advantage of the proposed coarse symbol timing offset detector presented herein is insensitivity to phase and frequency offsets and ISI. The proposed method uses pre-known data in the DVB-S2-like transmission stream to estimate the timing offset when a classical timing recovery scheme using feedback is not applicable due to large ISI. The proposed detector relies on pre-known symbols, but for modern communications designs, Turbo-codes, or LDPC codes with large frame length, are used which makes the insertion of pre-known symbols for frame synchronization mandatory. Also, for operation in low SNR, additional pilot information is often added to a signal to aid in demodulation.
The fine symbol timing recovery methods discussed herein help the symbol timing recovery system achieve and maintain lock under adverse conditions, including those used to increase the channel capacity of satellite channels, such as operating a lower SNR thresholds, reducing excess bandwidth, and when faster than Nyquist (FTN) signaling is used, which often make symbol timing recovery difficult.